1. Field of Invention
The present invention relates generally to data communication systems. More particularly, the present invention relates to systems and methods for accurately determining the frame format of a DS3 signal.
2. Description of the Related Art
The demand for data communication services is growing at an explosive rate. Much of the increased demand is due to the fact that more residential and business computer users are becoming connected to the Internet. Furthermore, the types of traffic being carried by the Internet are shifting from lower bandwidth applications towards high bandwidth applications which include voice traffic and video traffic.
DS3 traffic is a prevalent type of traffic in large networks. A DS3 signal generally has a bandwidth of approximately 44.736 megabits per second (Mbps), and may carry twenty-eight DS1 signals. DS3 line cards which support DS3 traffic allow routers within a network to be connected to high-speed DS3 leased line services. In general, DS3 line cards include ports that are used to receive DS3 signals.
A DS3 signal generally includes multiple frames which contain series of bits that are arranged as rows of data. FIG. 1 is a diagrammatic representation of a frame of a DS3 signal. A frame 100 in a DS3 signal includes 4760 bits that are divided into seven rows 104, or subframes, of data. The bits included in frame 100 include overhead bits 106, 109, 110 and payload bits. Payload bits include sets 108 of information bits. Typically, each set 108 of information bits includes eighty-four bits. Overhead bits 106, 109, 110 include frame boundary bits which enable payload bits to be extracted from frame 100 correctly. Frame 100 includes twenty-eight F-bits 109 and twenty-one C-bits 110, the latter of which may be used as stuff control bits for an M23 framing format to indicate how bits are stuffed in a frame, as will be appreciated by those skilled in the art. In other words, C-bits 110 may identify the contents of M23 stuff bits. C-bits 110 may also be used as parity bits, far-end alarming bits, remote loop back bits, and far-end error bits. F-bits 109 are framing bits which are needed to synchronized to a DS3 frame structure.
DS3 signals typically have three possible frame or framing formats, namely a C-Bit parity or C-Bit format, an M23 format, or an unframed format. A signal with a C-bit frame format generally uses C-bits, i.e., C-bits 110 of FIG. 1, to provide path parity information, as well as both local and remote alarm and control, including the ability to initiate remote loops. A signal of an M23 format generally includes a multiplexed scheme which provides for transmission of seven DS2 channels.
Properly identifying a frame format for a DS3 signal or DS3 traffic is generally necessary in order to enable a port on which the DS3 signal is to be received to be properly configured. When a port is configured or provisioned incorrectly, the payload of received frames may be corrupted as some bits may be interpreted as being payload bits when those bits are not part of the payload. By way of example, as part of C-bit usage, a frame that is of a C-Bit format generally includes a far-end loopback mechanism, while a frame that is of an M23 format does not use a loopback mechanism as a part of C-bit usage. Hence, provisioning a port to expect a signal of a C-Bit format when a signal of an M23 format is received may result in the signal being misread.
Typically, a first C-bit in a DS3 frame is considered to be an application identification (AIC) bit. In general, a standards compliant DS3 signal will have the AIC bit set to a value of ‘1’ when the DS3 signal is of a C-bit frame format, and set to a value of ‘0’ when the DS3 signal is of an M23 frame format. Hence, many systems may base an identification of a frame format upon the value of an AIC bit. FIG. 2 is a block diagram which illustrates a process of identifying a frame format of a DS3 signal. A frame format detection mechanism 202, which may be associated with a line card that is arranged to support DS3 signals, takes fixed overhead bits 204 as input. The fixed overhead bits 204 typically include the AIC bit. Using fixed overhead bits 204, frame format detection mechanism 202 provides a classification of a DS3 signal type 206 as an output.
With reference to FIG. 3, the steps associated with one conventional process of determining a DS3 frame format will be described. A process 300 of determining a DS3 frame format begins at step 302 in which F-bits and M-bits are obtained from a DS3 signal frame, or a frame included in a DS3 signal. More specifically, bits which are located in “standard” F-bit and M-bit locations are obtained. A determination is then made in step 304 as to whether the F-bits and the M-bits indicate that the frame format of the signal indicates that the signal is of an unframed format. As will be appreciated by those skilled in the art, the F-bits may be examined to determine if there is a repeating ‘1001’ pattern. When there is a repeating ‘1001’ pattern of the F-bits, then the M-bits may be examined to determine if there is a repeating ‘010’ pattern. If the F-bits do not have a repeating ‘1001’ pattern, then the indication is that the signal is of an unframed format. Accordingly, the signal is processed in step 306 as having an unframed format. Once the signal is processed as having an unframed format, the process of determining a DS3 frame format is completed.
When it is determined in step 304 that the F-bits and the M-bits do not indicate an unframed format, then the implication is that the DS3 signal is either of a C-Bit frame format or an M23 frame format. As such, in step 312, an AIC bit is obtained from the DS3 signal or, more specifically, the current frame of a DS3 signal. Once the AIC bit is obtained, it is determined in step 314 if the AIC bit indicates that the frame is of a C-Bit format. When it is determined that the AIC bit indicates that the frame is of a C-Bit format, the signal is processed in step 316 as having a C-Bit format, and the process of determining the DS3 frame format is completed. Alternatively, if it is determined that the AIC bit does not indicate that the frame is of a C-Bit format, then the frame is generally assumed to be of an M23 format. Hence, the signal is processed as having an M23 format in step 320, and the process of determining the DS3 frame format is completed.
Although using AIC bits to determine whether a DS3 signal has a C-Bit frame format, an M23 frame format, or an unframed format may be effective, the use of only the AIC bits may result in an inaccurate determination of a frame format. Legacy equipment, or equipment that was deployed before DS3 signal standards were ratified, and other equipment that is not standards compliant with respect to DS3 signals may use bits in a DS3 frame in a non-standards compliant manner. In some cases, equipment that is not DS3 signal compliant may set bits in a frame such that the first C-bit in the frame may not be an AIC bit, and may instead be used as a payload bit. As will be appreciated by those skilled in the art, the first C-bit in a frame is an AIC bit when the frame is of a C-Bit format. When the first C-bit in a frame is not an AIC bit, if the first C-bit is used substantially alone to determine a frame format, the determined frame format may be inaccurate. By way of example, a frame of an M23 frame format generally includes many values of ‘1’ in its payload. Hence, if the first C-bit in the frame is used as a payload bit and holds a payload value of ‘1,’ the M23 frame may be incorrectly identified as being of a C-bit frame format. When a frame is in a channelized M23 mode, i.e., when the frame is associated with DS1 signals that are combined to make a DS3 signal as opposed to being unchannelized with the payload of the frame being used in bulk, the first C-bit may be used as a stuff control bit, which may have a value of ‘1.’ When the frame format of a signal is incorrectly identified, a port that receives the signal may not be provisioned properly, and data contained in the signal may be improperly interpreted or reduced.
Many networks include legacy devices from a DS3 signal standpoint, devices which comply with DS3 standards, and devices which do not comply with DS3 standards. Since DS3 standards compliance varies from device to device in such networks, some signals within the network may have an AIC bit which is arranged to identify whether a signal is of a C-Bit frame format or an M23 frame format, while other signals may not actually have an AIC bit.
Referring to FIG. 4, a network which includes legacy DS3 devices, non-standards compliant DS3 devices, and standards compliant DS3 devices will be described. A network 400 includes devices 404a-f, e.g., routers. Devices 404 include standards compliant DS3 devices 404a, 404b, as well as a non-standards compliant DS3 device 404d and a legacy DS3 device 404c. When standards compliant DS3 device 404a sends a signal of a C-Bit frame format, the AIC bit of each frame in the signal will generally be set to a value of ‘1,’ although it may be possible that by the time the signal is received, e.g., at standards compliant DS3 device 404b, due to bit errors, the AIC bit in one of the frames may be incorrect. As a result, standards compliant DS3 device 404b may incorrectly identify the frame format of the received signal.
Further, as previously mentioned, legacy device 404c and non-standards compliant device 404d may transmit DS3 signals with frame formats that may not be accurately identified. Since the determination of the frame format of DS3 signals is typically based on fixed overhead bits such as an AIC bit, when the overhead bits are not set according to ratified standards for different frame formats, incorrect determinations of a frame format are likely to occur.
Overhead bits within a DS3 frame of a particular frame format may be inconsistent with that frame format due to at least some of the overhead bits not being used in an expected manner. As discussed above, some devices may store payload information in bits which are typically designated as being overhead bits. When bits in a frame which are expected to be overhead bits are not actually used as overhead bits, a device which receives the frame may falsely trigger a frame format, e.g., an M23 frame may incorrectly be detected as a C-Bit frame.
In addition, transmission errors may cause even overhead bits which are used in an expected manner to be incorrectly received. By way of example, transmission errors may cause an AIC bit in a frame to be changed from one value to another, thereby causing the wrong frame format to be detected.
Therefore, what is needed is a method and an apparatus which allows DS3 frame formats to be more accurately determined. That is, what is needed is a robust system which increases the likelihood that DS3 frame formats may be accurately detected, particularly in systems which may be subject to transmission errors, and include either or both legacy DS3 equipment and non-standards compliant DS3 equipment.